Yikes, I got lost somewhere since last time. I'm overdue for Part II, so here it is.
Previously, I discussed the sun-space parameters that set up the success for the world. As before, it is usually just simpler to assume an earth-like world: year-long revolution around the star, 24-hour day, 23.5 degree tilt for the seasons.
As an aside, you are correct: this does involve a lot of assumptions. But the world is supposed to flavor and add to your story, not be the driving factor (unless it is). Assumptions are okay, as long you are internally consistent. (If you think that's wrong, ask your favorite physicist if they have ever assumed a spherical cow.)
1) Start with the discussion of Hadley cells. Hadley cells basically postulate a conveyer-belt effect of airflow in the atmosphere: heating and rising at the equator (the spot of most consistent heating), settling around thirty degrees north and south of the equator, and rising again around sixy degrees north/south. What this basically means is that you have continual low pressure at the equator – meaning lots of convection (i.e., rain) – and high pressure in the sub-tropics (+/- thirty degrees latitude form the equator). More on that in a moment. You can read more on Hadley cells here.
2) We move on to the Corilois force. Again, you can read up on more of the physics here, but the net effect involves the rotation of the Earth resulting in a right-hand curvature of motion in the atmosphere. This is the effect that gives us the different prevailing winds at different latitudes. Between roughly thirty and sixty degrees (on the north side, that encompasses most of North America and Europe), the prevailing winds are westerly – meaning that weather systems move from west to east.
It's worth pausing here to revisit the first entry on the subject. You can now see how some of those interactions might change. Say you want your sun to rise in the west. Spin your world in the opposite direction, the Corilois force moves in the opposite direction and BOOM, the prevailing wind will be the opposite of our world.
Okay, so now that we've done that, what next? You need to place your major landmasses and determine the geographic features that aren't dependant on weather patterns. In this case, I would be talking about mountain ranges and dry land versus ocean land. In the large scale, mountains work independent of rain; plate techtonics drive the formation of mountains and either raise continents or drive them into the drink. Barring supernatural/magic drivers or cataclysmic events (a la the movie 2012 or the mountain being thrown on the city of Istar in the Dragonlance series ), land mass formation is hyper-slow and once you place them, feel free that throughout your history, the land won't change that much.
Now, you've placed your land masses, determined prevailing winds. What next?
In the absence of land masses, the currents would basically follow the winds. You don't have to get hyper-anal about this, but just use real-world guidance to come up with something realistic. (This will become important in part III.) For example, around North America, the water tracking from the equator up to the northwest enters the warm Caribbean and Gulf of Mexico, flows up the east coast, then across the Atlantic, creating the … wait for it … Gulf Stream, the warm water that keeps western Europe somewhat warmer than other locations at that latitude. Conversely, on the west coast, cold water flows down from northern Pacific, which is why the water off San Francisco is significantly colder than off Virginia Beach, even though they are at approximately the same latitude and receive about the same amount of solar radiation in a year.
(Caveat: I am not an expert on current dynamics, so like I said, just looking for a common-sense answer.)
Okay, so we have the rock in space, with some parameters set. Now we've added prevailing winds, land masses, and some water flow. What's next?
That lifeblood known as water. What does it do?
That's what we'll discuss next time.